1. Field of the Invention
The present invention relates to an accumulation time control apparatus for sensors which are used in a focal point detecting apparatus having a plurality of distance measuring visual fields or the like.
2. Related Background Art
Hitherto, various methods of controlling an accumulation time of an AF sensor have been proposed. However, in most of the cases, there is generally used a method comprising a combination of both of a control method of terminating the accumulating operation when a charge accumulation amount has reached a predetermined signal level due to the automatic gain control (AGC) and a control method of terminating the accumulating operation when a predetermined maximum accumulation time has occurred even if a charge accumulation amount doesn't reach a predetermined signal level. A reason why two kinds of control methods are used is because a dynamic range of the luminance of an optical apparatus such as a camera or the like is extremely wide and cannot be completely controlled by one method.
The conventional known method will now be simply described hereinbelow with reference to the drawings.
FIG. 1 shows an AF optical system of the typical double image phase difference detecting type.
In FIG. 1, a field lens FLD has the same optical axis as that of a photographing lens LNS whose focal point is to be detected. Two secondary image forming lenses FCLA and FCLB are arranged at positions which are symmetrical with respect to an optical axis behind the field lens FLD. Sensor arrays SAA and SAB are arranged at positions further behind the lenses FCLA and FCLB. Diaphragms DIA and DIB are arranged near the secondary image forming lenses FCLA and FCLB. The field lens FLD forms an image of an exit pupil of the photographing lens LNS almost onto pupil surfaces of the two secondary image forming lenses FCLA and FCLB. Thus, the light fluxes which respectively enter the secondary image forming lenses FCLA and FCLB are light fluxes which were emitted from the regions having the same area which are not mutually overlapped and correspond to the secondary image forming lenses FCLA and FCLB on the exit pupil surface of the photographing lens LNS. When an air image formed at a position near the field lens FLD is again formed onto the surfaces of the sensor arrays SAA and SAB by the secondary image forming lenses FCLA and FCLB, the positions of two images on the sensor arrays SAA and SAB are changed on the basis of a deviation of the position of the air image in the direction of the optical axis. Therefore, a focal point state of the photographing lens LNS can be known by detecting a deviation amount of the relative position of two images on the sensor arrays.
FIG. 2 shows an example of photoelectric conversion outputs of two images formed on the sensor arrays SAA and SAB. The output of the SAA assumes A(i) and the output of the SAB assumes B(i). As the number of pixels of each sensor, at least five pixels are needed and, preferably, tens or more pixels are necessary.
As a signal processing method of detecting an image deviation amount PR from the image signals A(i) and B(i), applicant herein has proposed methods as disclosed in Japanese Laid-Open Patent Application No. 58-142306, Japanese Laid-Open Patent Application No. 59-107313, Japanese Laid-Open Patent Application No. 60-101513, Japanese Patent Application No. 61-160824, and the like.
The photographing lens can be set into an in-focus state by adjusting a focal point of the photographing lens on the basis of the image deviation amount obtained by the methods disclosed in the above-cited Japanese references, which have respective counterparts in U.S. Pat. No. 4,559,446, U.S. Pat. No. 4,559,446, U.S. Pat. No. 4,618,236 and U.S. Pat. No. 4,812,869.
The accumulation times of the AF sensors SAA and SAB are controlled, for example, in the following manner. In FIG. 3, control sensors SAGCA and SAGCB are arranged at positions adjacent to the sensor arrays SAA and SAB and observe almost the same object portion as the distance measuring visual field images. Outputs of the control sensors SAGCA and SAGCB are added by an adder ADD and an addition signal is supplied from the adder ADD to comparators COMP.sub.1 and COMP.sub.2.
A potential which is compared with the addition output by each comparator is obtained by dividing a reference potential V.sub.ref. The comparator output potentials correspond respectively to the upper level and lower level in FIG. 4. Output signals of the comparators are always supplied to a control circuit CONT.sub.2 and are referred to at a certain point in time. A control signal to control a control circuit CONT.sub.1 is generated from the control circuit CONT.sub.2. The control circuit CONT.sub.1 receives the control signal from the control circuit CONT.sub.2 and transfers and reads out the photo charges of the AF sensors to a transfer channel TRANS through a gate GATE in order to terminate the accumulation of the charges by the AF sensors. The read-out charges are supplied to an operating circuit through an amplifier of a designated gain.
The control circuit CONT.sub.2 receives two kinds of time designation pulses BTIME and TMAX and executes the following operation.
When the upper comparator COMP.sub.1 is turned on before the BTIME pulse arrives, that is, when an accumulation amount of the control sensor has reached the upper level in FIG. 4, the accumulation is finished at that time point and a signal is generated so as to select an amplifier of a low gain in the control circuit CONT.sub.1. The output signal of the adder ADD changes in accordance with a straight line L.sub.1 in FIG. 4 with the elapse of the accumulation time and the accumulation is finished at time T.sub.1 before the time point of the pulse BTIME.
If the accumulation amount of the control sensor doesn't reach the upper level before the arrival of the pulse BTIME, the gain of the amplifier in the control circuit CONT.sub.1 is selected in response to the pulse BTIME. That is, if the accumulation amount of the control sensor is higher than the lower level at that time point, the low gain is selected and the accumulation is continued until the accumulation amount is equal to the upper level of the comparator. The output of the adder ADD changes in accordance with the straight line L.sub.2 in FIG. 4. The accumulation is finished at time T.sub.2. On the other hand, if the accumulation amount is lower than the lower level, the accumulation is continued until the accumulation amount reaches the lower level as shown by a straight line L.sub.3. The accumulation is finished at time T.sub.3. At this time, the control circuit CONT.sub.2 instructs the control circuit CONT.sub.1 to use an amplifier of a high gain. A ratio of two gains is set to a value which is equal to a ratio of two comparison levels.
In the above description, it is not always necessary to use two kinds of gains. In a sensor pixel construction as shown in FIG. 5 in which an amplifying transistor is provided for each pixel of an AF sensor array, by using a double emitter construction as the above transistor and by commonly connecting one of the emitters of each transistor, the accumulation time can be controlled by using the image itself in the distance measuring visual field.
On the other hand, in recent years, there has been developed a method of automatically detecting a focal point whereby an in-focus state is detected and adjusted in a wide range in a picture plane by setting a plurality of distance measuring visual fields. For instance, an in-focus state detecting apparatus for a single-lens reflex camera as shown in FIG. 6 has been proposed. In the apparatus, the light flux for AF which was reflected downwardly by a submirror SUBM enters a visual field mask VMSK having three different distance measuring visual fields through field lenses FLD.sub.1, FLD.sub.2, and FLD.sub.3. After that, the AF light fluxes are transmitted via mirror reflecting members M.sub.1 and M.sub.2 to extend a length of optical path and enter a pair of image reforming lenses FCLA and FCLB and images are again formed onto the surfaces of the sensors. Three pairs of sensor arrays, that is, a pair of sensor arrays SAA.sub.1 and SAB.sub.1, a pair of sensor arrays SAA.sub.2 and SAB.sub.2, and a pair of sensor arrays SAA.sub.3 and SAB.sub.3 are used to individually receive the reformed optical images which derive from three visual fields of the visual field mask VMSK.
Even in such an AF apparatus for detecting in-focus states at a plurality of points in the picture plane, a method of controlling the sensors is the same as that in the conventional apparatus. The accumulating operations of all of the sensor pairs are started together and the accumulation control as mentioned above is executed for each sensor pair. There have been proposed several techniques with respect to a method whereby the AF arithmetic operations are executed from the sensors whose accumulating operations were finished and an in-focus state of the camera is obtained by applying an algorithm to a plurality of in-focus detection values. For instance, there is a method whereby among three distance measurement results, the result corresponding to the position which is closest to the camera is selected. The above method is based on the idea such that the main object exists at the nearest position and the images existing at relatively remote positions other than the main object are the background. There is also known a device for weighting in a manner such that the selection algorithm of the distance measuring points is changed in accordance with a focal point distance of the photographing lens and on the side of a short focal point, the selection of the central visual field is made easy or the like.
However, if the control of the multi-point AF sensors is executed by the conventional method, there are the following problems.
If the method of controlling the accumulation of the sensors as mentioned above is used, generally, in the case where there is a luminance difference in the picture plane of an object, the sensor output of the distance measuring point at which a portion of a low luminance is seen does not reach a predetermined level, so that the accumulation is not finished, a desired time of the AF operation becomes remarkably long, and the operability of the apparatus is ineffective.
On the other hand, a contrast constant control method has been known as a method of controlling the accumulation time. In this case, even in a space which is uniformly illuminated, contrasts of the light intensity distributions on the AF sensors differ depending on a pattern of an object, so that the accumulation times of the sensors having different distance measuring visual field differ. For instance, in the case of the sensor which observes a wall or a cloth of a solid color, even if the accumulating operation was executed for a long time, a contrast enough to reach a comparison level is not obtained. Therefore, even in a bright state, the maximum limit accumulating operation is soon performed. In the ordinary passive AF operation, it is necessary to execute a few sensor accumulating operations such as initial light reception of a light image, light reception of a light image for confirmation after the optical system was driven, confirmation after a fine correction in the case where an in-focus state is not derived as a result of the confirmation of the light image, and the like. If there is a sensor whose accumulation time is particularly long due to the causes as mentioned above, a time until the in-focus state is derived becomes remarkably long and the operability of the optical equipment is ineffective. Moreover, the distance measuring visual field which is seen by the sensor whose accumulation time is especially longer than those of the other sensors has an extremely low luminance and an extremely low contrast. Therefore, in many cases, an object to be photographed is not the main object. In other words, the operability is ineffective because of the distance measurement in which a possibility such that the result is eventually unnecessary is high.